Through the tradition of Shin Megami Tensei series, the Demi-fiend is a silent protagonist, where his personality is based upon players' actions and decisions. As per the franchise's standards, he is a blank slate at the very beginning of the game, unsure of the role he is to play in the world that is to come; creator or destroyer, master or servant. This is most clearly seen when Kagutsuchi scans his heart at the moment of the Conception. The divine being finds nothing there, no defined ideas or motives, and decides he needs to find his own ideology and Reason in the Vortex World.

During the game, his personality develops further depending on the choices of the player, molding him into a follower of either Reason or a person who rejects all of them, either through weakness, a true desire to see the world reborn, or an honest wish to end the eternal cycle of death and rebirth. The definitive moment is when Kagutsuchi, at the entrance of the Tower of Kagutsuchi, again scans his heart and determines what lies within.

He is, however, consistently, defined by his choice to act; it is impossible for him to remain doing nothing while others move behind his back. At least initially, he is a relatively trusting and naive person; this is a trait Jyoji Hijiri uses and abuses to convince him to be his errand boy in the Vortex World to the point of sending him to deactivate the Nightmare System and confront both the Assembly of Nihilo and the Mantra Army. Isamu Nitta remarks upon said trait in an insulting fashion upon being released from Kabukicho Prison as well. Isamu would later expand upon this and note how it could very easily be turned into a flaw.

This trait also makes him a relatively predictable power, provided he can be forced into a situation where someone can directly profit from his intervention (as seen when Isamu captured Hijiri and forced the Demi-fiend to destroy the Amala Temple demons to see him again).

Among those who help shape the Demi-fiend's personality is Aradia, who confronts him openly about how far he was willing to sacrifice his everything in pursuit of the world and forced him to either confront his fears or admit them. Others include: Gozu-Tennoh, who inquires about his power lust; Lucifer and the Fiends, who stand poised to create a new demon of Chaos; Metatron, who seeks to avert Lucifer's goal; Sakahagi, who confronts him with his demonic nature; Futomimi, who embodies a more rational and human side; Hikawa, who offers the temptation of a world ruled by serenity and free of pain; and Yuko Takao, who represents the last thin strands of the person he used to be.

He is also desperate for reliable information about the Vortex World and how to manipulate it (desperate enough to be swayed by Lucifer, following his orders regarding the Candelabra): Beelzebub notes when fought in the Fourth Kalpa of the Labyrinth of Amala that he was following Lucifer's plan without having any clear idea of what exactly it entailed, getting snippets of information from enemy and ally alike, lightly mocking him for such an irrational stratagem.

Due to different choices in the course of the game, his personality can be developed further, revealing character flaws and strengths, changing others' viewpoints of him and affecting the possible ending.

In his brief appearance in the Shin Megami Tensei IV Apocalypse DLC, the Demi-fiend is slightly impatient and imposing, as well as having a lust for battle to a certain degree, evident by his want of a rematch with Lucifer.

The protagonist of Shin Megami Tensei III: Nocturne was originally a Japanese high school student, who agreed to meet up his friends to go to the Shinjuku Medical Center and visit one of their teachers, Yuko Takao. En route to the hospital, he encounters several accounts about a strange cult, specifically the Cult of Gaia that had been spreading "prophecies" about the imminent advent of the Conception for a while now. The protagonist later runs into a journalist named Jyoji Hijiri during a stop to Yoyogi Park, where the group was supposed to meet (Chiaki eventually called to tell him that they already went ahead, and he should just follow). Unintentionally hearing the phone conversation, Hijiri mentions that he was also on his way to the hospital, and reveals that he is currently investigating the activities of Gaea; or more specifically, the case regarding the massacre that occurred within the park, which he believes to be the work of creatures that were not of this world. He then proceeds to give the protagonist a soon-to-be-published issue of "Monthly Ayakashi Magazine".

Upon meeting up with Chiaki in the hospital lobby, the protagonist searched the seemingly abandoned rooms in the second floor for Isamu, who went up earlier to find Yuko (For her part, Chiaki opted to stay in the lobby and read the magazine instead). With their search bearing no fruit and suspecting that something was amiss, Isamu proposes that they split up to cover more ground, and gives the protagonist an access card so that he can go through the restricted basement area.

What he finds within is a truly frightening sight. There are scattered bloodstains around the corridors, and some of the rooms within the facility have dubious setups evident of cultist activity. The protagonist later encounters a man named Hikawa in one of the rooms, who sees the protagonist as a potential hindrance to his purposes after having seen everything. Hikawa proceeds to summon a demon with intentions to kill the protagonist, but Yuko arrived in the nick of time to stop him, threatening to exclude herself from Hikawa's plans if he harms her student.

As Yuko leaves the room, she asks the protagonist to follow her to the rooftop. He soon encounters a young blond child and an old nursemaid in mourning garb while walking to the elevator, both of whom disappear when he tries to approach them.

Upon reaching the rooftop, Yuko confesses to him that her sickness was just a ruse. She actually led the protagonist to the hospital so that he could survive the Conception, as it is the only place that would be relatively safe. Yuko tells the protagonist to stay calm for what is soon to unfold, requesting of him to find her afterwards so she could explain everything, resolving to become his strength.

No sooner has she said those words when an ominous wind blows, and a concentration of negative energy slowly builds. Every living being outside is killed; black flashes of lightning obliterate random structures, and the earth begins to rise, wrapping itself around a bright blue sphere of energy suspended at an unreachable height. The world has turned itself inside out, and the sky can no longer be seen. This is the Vortex World, the apocalyptic state of chaos where the old world ends, awaiting a new beginning.

As he fades in and out of consciousness, the protagonist encounters the young child and his nursemaid again. To allow the protagonist to survive in a now demon-ridden world, the nursemaid subdues him and the child drops a Magatama — the essence of demonic power — into the protagonist's eye; allowing the parasite entity to merge with him, resulting in the birth of the Demi-fiend — a being with the body of a demon and the heart of a human.

The protagonist's body has now been altered, with glowing tattoos emerging on his skin and a horn that grew from his nape as proof of ingesting the Magatama. He now has an incredible destiny on his shoulders: He is to either create the new world by supporting a Reason, deviate from the process completely to return the world to normal, or completely shed his humanity by embracing his demonic self and end the process of the world's destruction and rebirth.

As a result of his parasitic fusion with the Magatama, the protagonist becomes capable of performing a variety of demonic feats, ranging from using powers granted by the Magatama to conversing with other demons. He can gain information and possibly recruit them to his side as allies if he utilizes proper skills of negotiating, taking into account the demon's behavior and the phase of Kagutsuchi, which significantly affects them in the Vortex World.

After his transformation, the protagonist awakens in the morgue of the now demon-ridden Shinjuku Medical Center. He eventually meets a Pixie, who agrees to work with him towards finding the keycard to the annex and escape the hospital in exchange for taking her to Yoyogi Park, which is now the territory of her clan. With Pixie's help, as well other demons he would successfully persuade to join him, the protagonist defeats Forneus, who had declared itself the new master of the hospital and was blocking the only open exit (off-hand, there is mention of a human who was able to slip past Forneus prior to the encounter).

In the Maniacs version, after being sidetracked in the Amala Network while on the way to Ginza, Demi-fiend was transported into the Labyrinth of Amala. Looking into a strange peephole, he meets an old man in a wheelchair and a lady in black mourning garb standing on a stage that had a set piece of a mansion's living room which looked out of place amidst the unsettling environment. The latter lauds Demi-fiend's strength and gives him the Candelabrum of Sovereignty, requesting him to seek out the other candelabra which were stolen by the Fiends. If Demi-fiend accepts this request, he must face the many fiends under Lucifer's command who will force him into battle hoping to obtain his Candelabrum, including a demon hunter named Dante, but if he declines, the Labyrinth of Amala will be forever closed to him.

Throughout his journey in the Vortex World, the protagonist slowly realizes his role in the creation of the new world, and how he has significant influence on which of the established Reasons created by various factions and tribes will come into being. Even if he gains new allies and minions, he also slowly loses his friends, Isamu and Chiaki, who become hardened after realizing that they possess the potential to shape the world to their liking. Aside from them, many other individuals hope to establish the Reasons they've created in the Vortex World as well, and would actively seek out the protagonist's cooperation.

The only neutral human who has been an ostensible ally of the protagonist in the person of Hijiri is eventually corrupted by his newfound knowledge of the workings of the Amala Network and tries to pursue the path of creation as well, but ends up being abducted and killed by Isamu, used as a sacrifice to activate the release of Magatsuhi in the Amala Temple for his sponsor god, Noah. Chiaki on the other hand, invades Asakusa and kills Futomimi, the leader of the Manikins, using the Magatsuhi in Mifunashiro to gain the blessing of Baal Avatar. Yuko Takao, who promised to protect the protagonist, secretly tries to create her own Reason while working under Hikawa, but is led astray by Aradia's deception and her own lack of composure. She later dies as Hikawa summons his sponsor god Ahriman by releasing the stored Magatsuhi in the Diet Building.

As she slowly fades away, Yuko tells the protagonist that even if she failed in pursuing her ideal world, she is not really worried, for she has confidence that the protagonist can possibly create the world as he would see fit even without external influences. Yuko passes the Yahirono Himoragi on to him, for aside from being able to hold a significant amount of Magatsuhi, it also serves as a key to have the Tower of Kagutsuchi descend. With the time of creation drawing near, Kagutsuchi beckons the holders of the Reasons to stand before him after proving their worth by defeating the opposing factions and offering the three stones.

Once the protagonist enters the Tower Of Kagutsuchi, his heart will be read, and whatever path of creation he supports will be final.

If the protagonist supports Isamu's Reason, he will not fight Noah in the Tower Of Kagutsuchi, but the latter will instead be bested by Baal Avatar. Upon defeating Kagutsuchi, the protagonist sees a message written on a stone tablet from the late Isamu, who relinquishes the responsibility of creating the world to the former based on his ideals of individuality, and hopes that he will be born again in a world where his life would no longer be dictated by anyone.

The prerequisites of unlocking the ending for Musubi requires the protagonist to support Isamu's resolutions every time they meet up.

The protagonist can choose to ally with Hikawa's Reason, a philosophy based on stillness, unity and suppression of human desires. In the end, Hikawa accepts the fact that his part is merely the "baptist" of this world and acknowledges the protagonist as the true creator according to what was written in the Scripture Of Miroku. For his part, Hikawa can now sit back and watch contentedly as the world he envisioned will soon come into being.

The prerequisites of unlocking the ending for Shijima requires the protagonist to support Hikawa's resolutions every time they meet up.

If the protagonist supports Chiaki's Reason, they will still fight in the Tower Of Kagutsuchi, because Chiaki wants to see if he is strong enough to facilitate the creation of the world where might makes right. In the end, Chiaki appears as a ghost, and admits that while she thought at first that she'd be the one to rule the world, it seemed to be her destiny to choose the protagonist instead, having proven himself stronger. Before fading away, Chiaki guides the protagonist on where he is supposed to go, and asks him to say hi to the newly-created Yosuga world for her.

The prerequisites of unlocking the ending for Yosuga requires the protagonist to support not only Chiaki's resolutions every time they meet up, but also Gozu-Tennoh's, whose ideals of ruling the world with strength early on would eventually mirror Chiaki's own philosophies.

Despite being forbidden to create his own Reason, the protagonist does not necessarily have to support any of the Reasons of Musubi, Shijima, or Yosuga. He can decide to forego the process of creation and leave the Vortex World as it is, a barren, desolate land populated by demons. This is the 'Demon' Neutral Ending, in which the protagonist does not have to face Katsuguchi, having been banished and cursed by the latter for all eternity for facilitating the death of this world as the incarnation of bane itself.

The blond child later appears before the protagonist, lauding his resolution of creating a millennial kingdom suitable only for demons. While Kagutsuchi has cursed the protagonist, the blond child would bless him; he is now a demon of darkness, having been born at the expense of the past world. Chaos will reign for years to come until the Conception occurs once again.

The prerequisites of unlocking the Demon Path ending requires the protagonist to reject all three Reasons or support more than one of the Reasons. Furthermore, he must respond to Aradia's questions with fear.

If his connection to the old world is strong enough, the protagonist can opt to turn Tokyo back to how it was before, provided that he does not fully support any one of the Reasons or succumb to his demonic side. In the Freedom path, the protagonist faces a furious Kagutsuchi who accuses him of being seduced by freedom, similar to how the fallen angel was before. After the protagonist proves his strength in the end, Kagutsuchi laments how absurd it is that his power would be used not to create a new world, but to revert it to its original state as a world of suffering.

The protagonist's demonic powers will wane a little (evident by the loss of his demonic skin), but they will not fade. The shape of the world has normalized and every human being is reincarnated, including Chiaki, Isamu, Hijiri, Yuko, and even Hikawa. The protagonist would then find himself back in his room with Isamu urging him to get going so they won't be late in meeting up with Chiaki at Yoyogi Park. Life truly has returned to normal.

In an e-mail message sent to the protagonist, Yuko tells him that she's feeling better and would leave the hospital soon. She also apologizes for her summary judgment of humanity's supposed degeneration, and admits that her jaded views were out of place. She acknowledges the protagonist's strong resolve so much that she became inspired to continue living this life to the fullest, regardless of how the world will turn out in the future. Yuko's message ends with a heartfelt "Thank you".

Lucifer would later remark that the protagonist's choices that led to recreating the world of freedom are reminiscent of his own. However, since the act is in essence a defiance of God's will, the former advises the protagonist to stay strong, which could possibly mean that he should keep his demonic powers in preparation for the day that the "true enemy" would also curse his existence.

The prerequisites of unlocking the Neutral ending requires the protagonist to reject all three Reasons or support more than one of the Reasons. Furthermore, Demi-fiend must respond to Aradia's questions with courage.

In the Maniacs version, the Demi-fiend can choose to ignore all of the Reasons and Alignments altogether. To do so, the Demi-fiend must accept and fulfill the old man's request of retrieving all the Candelabra and venture into the deepest area of Labyrinth of Amala. God would soon fear the Demi-fiend's vast capabilities and Lucifer's influence towards him. At the entrance to the Second Kalpa, Metatron will relay God's warning to the Demi-fiend to stop meeting up with the dark angel. Regardless of his answer, the Demi-fiend can still opt to go into the Labyrinth.

Deep in the Third Kalpa, the Demi-fiend will be forced to play a cat-and-mouse game with the demon hunter Dante, who resets door switches upon tagging the Demi-fiend and guards the exit when all the switches have been activated. In the last section, they will fight once more, and Dante will voluntarily give up the Candelabra to the Demi-fiend upon his victory, dismissing it as "nothing but junk".

In the Fourth Kalpa, the Demi-fiend will encounter Beelzebub upon reaching Hell's Hall. He promises to lend assistance if he passes the test of strength by fighting him as his true form, the "Lord Of The Flies". He is then unlocked for fusion upon defeat, bestowing his power in both humanoid and true forms as one of the Demi-fiend's minions. Metatron is enraged that the Demi-fiend chose to ignore his warnings, and decides to try and kill him later on at the Fifth Kalpa.

Should the Demi-fiend prevail against Metatron and manage to reach the deepest area of the Fifth Kalpa to meet up with the old man, the lady in black will express gratitude towards the Demi-fiend's efforts. For the sake of the demon race, the Demi-fiend has willingly thrown away the last vestiges of his humanity to become an agent of chaos, completely allying himself with the 'dark angel' in the battle against the absolute one. She also tells the Demi-fiend that he will soon be reborn, this time as a True Demon. But before he is totally accepted into the fold, the Demi-fiend must prove his worth by thwarting the respective ambitions of the three holders of Reason, as well as the main obstacle in the form of Kagutsuchi itself.

After the Demi-fiend receives a surge of demonic powers from the old man, the platform he was standing on sinks into the red waters. The old man looks at him and smiles, having succeeded in his plans of indirectly creating the strongest demon among demons.

The Demi-fiend awakens once more in the Shinjuku Medical Center morgue, and finds that the very first Magatama he ingested has a new power which will allow his physical attacks to bypass most kinds of protection, a fearsome ability befitting his new identity. As a True Demon, Demi-fiend can no longer accept the influences of all the other Reasons, as he is now set on the deviant path as a servant of darkness.

The 'True Demon' Path ends with the Demi-fiend killing Kagutsuchi, freeing all the worlds from the control of God in the process. With the concept of time now dead and the cycle of creation put to a complete halt, the universe totally collapses and everything is thrust into darkness. Lucifer will then reveal that he has been watching the Demi-fiend all this time in the guise of both the young child and the old man. While he is pleased with the results, it remains to be seen if the Demi-fiend is ultimately capable of leading the way to vanquish the true enemy. Lucifer assumes his real form as the fallen angel and thrusts the Demi-fiend into battle to measure the true limits of his dark power.

Emerging victorious, the Demi-fiend becomes the secret weapon who will tip the balance of Light and Darkness, champion of the legions of chaos in the final battle against the Great Will.

The protagonist is given the name Naoki Kashima in the drama CD. He was close to Chiaki Tachibana and was believed by others that he would have begun dating her had the Conception never happened. He is a confident and independent person before the Conception. It is explained that Naoki refuses to tolerate bullies and that he does not respect the weak. He believes that a person should stand on their own feet to create their own destiny. Lucifer created the second Demi-Fiend - who is implied to be a full fiend - to act as a scapegoat for Kagutsuchi to destroy in the place of Naoki, having known what path both fiends would choose from the very beginning.

Sporting a new model, the Demi-fiend appeared as an event boss for Shin Megami Tensei IMAGINE's fifth anniversary as one of Lucifer's "Great Generals" alongside Beelzebub, Michael, Jeanne D'Arc, Shiva, and Vishnu.[1] Unlike his fellow event bosses, he is not recruitable. In his event fight, he summons the same six demons as he does in his ultimate boss fight in Digital Devil Saga. During this event and later through trading with the event NPC Saien, players could obtain cameo rings to temporarily change their appearance completely into the Demi-fiend's.

In the alternate Shinjuku Babel's Abyss Tower instance, Demi-fiend reappears as a stronger permanent boss with the title King of Chaos Demi-Fiend. Unlike the anniversary event variation, this Demi-fiend is completely insusceptible to ailments, resists physical, elements, drains almighty, and does not summon any demons for support. Upon defeat he simply gets back up and disappears offscreen, leaving it ambiguous as to what happens to him. While all bosses in the Abyss Tower have a chance of dropping the Amala Ring, defeating Demi-fiend is the only source for the items that upgrade the ring -- Satan, Kailash, Nirvana, and Miasma.

The Demi-fiend appears in Digital Devil Saga: Avatar Tuner as a powerful Ultimate Boss, who can only be challenged by starting a New Cycle+. He can be found in the Anahata underground dungeon. Aside from using altered special attacks which have gained a high critical rate, he possesses a modified version of Gaea Rage, which is now a devastating Almighty-type attack that can deal anywhere between 3000-30000 damage on all characters depending on their stats. Should Serph's party enter the battle with skills or equipment that nullify any of his attacks (e.g., Monotheist Mantra), the boss will start the battle with Gaea Rage, killing all present characters instantly. The only forms of protection that can be used are resist skills, such as resist physical. He is accompanied by two demon minions, and will summon a new minion to replace anyone who has been defeated. The demons that the ultimate boss summons are as follows:

For every two demons summoned, the boss will also use Gaea Rage. One of the ways to survive it, aside from dodging normally, is to have all characters endowed with low luck stats along with Null Sleep, a skill granting a 100% dodge rate against all attacks while sleeping. Finally, they must allow themselves to be hit by Dormina (cast by Pixe, Titania, and Parvati); Gaea Rage will miss every sleeping character.

The Demi-fiend possesses 18,000 HP, but when it drops to half, one of his demons will use Mediarahan to fully heal him, effectively boosting his total to 27,000. However, this only happens once for the duration of the fight.

Upon defeating him, the party will find themselves back in the Anahata dungeon, awakening in the chamber and noticing a dying message written on the floor: "Death's vastness holds no peace. I come at the end of the long road. Neither human, nor devil... all bends to my will". The Amala Ring will then be unlocked; it is an accessory that can only be obtained in the sequel. It is similar to the MasakadosMagatama, as it gives +10 to all stats.

Demi-fiend's difficulty has made him an infamous boss fight, appearing as #1 on multiple Top-10 most Difficult RPG Bosses, and many other gaming websites also consider him the most difficult as well. The Random Battle Music from Nocturne plays during his encounter, generally believed to represent his view of the battle as an everyday, random encounter.

The Demi-fiend appears as a temporary ally in the DLC, "Messiahs of the Diamond Realm." Alongside the Hero, Aleph, and Flynn, he had also died in his world due to YHVH's schemes, considering them too dangerous. In his world, he had been killed by Lucifer after destroying Kagutsuchi, but was brought back in the Diamond Realm by Stephen, in order for them to regain their lost strength. He travels back with the intent of winning this time.

The protagonist's main skills are gained via the player's choices with the Magatama. While normal demons are restricted to using specific skills based on their external features, his powers allow for a certain degree of flexibility. For example: Heat Wave and other blade-based attacks are generally learned/inherited only by demons who wield weapons, but the protagonist can learn to use them even if he does not have any by default. This is done by projecting an energy sword to execute the attack.

The protagonist also has access to skills that are exclusive to him, such as Magma Axis, Spiral Viper, Freikugel and several others. When using some of these skills, the protagonist's skin darkens and his tattoos seem brighter in contrast.

He also bears the distinction of having an innate conversation skill which does not take up a skill slot, namely Talk. As the main character, he is also the only one capable of summoning demons (unless he has a demon present with the Beckon Call skill) and using items during battle.

Kazuma Kaneko has said in an interview that the concept of the Demi-fiend was inspired by the band the Red Hot Chili Peppers. Additionally, Demi-fiend's tattoo design is intended to convey the image of a shaman.[2]

The protagonist's original hooded jacket has a demon motif on the sleeves, as mentioned by Isamu at the beginning of the game. The said motif is based on the 8-bit demon sprite used in many Megami Tensei games, denoting demon status and numbers.

In the New Cycle, the protagonist can opt to start the game with a leather jacket, which is his outfit in the vanilla Japanese version of Shin Megami Tensei III: Nocturne. The change only lasts until the Conception, but will alter his base stats once he becomes Demi-fiend.

The protagonist's eye color differs depending on his current form: his eyes are grayish-blue while still human, yellowish while a half-demon, and if he chooses to become a true demon in Maniacs, his eye color becomes red in the end, the symbol of his lost humanity.

However, in-game, Demi-fiend's eye color still retains its grayish-blue shade instead of turning yellow, unlike Chiaki and Isamu when they become pseudo-fiends.

In his boss battle in Digital Devil Saga: Avatar Tuner, the random encounter theme of Nocturne can be heard in the background. Additionally, his dying message is implied to rebuke the Great Will. A side-by-side comparison of the Demi-fiend's message to Kagutsuchi's reveals an inverse response to the death screen of Nocturne.

In Digital Devil Saga: Avatar Tuner 2, Satan tells the player that a heretic is trying to destroy the balance of the universe, just like the protagonist of that game before the battle. He is most likely referring to Aleph from Shin Megami Tensei II, as the Digital Devil Saga duology contains multiple references to the events that took place in that game, however, it is also suspected that he may be referring to the Demi-fiend.

His human clothing and hairstyle are promotional items for male avatars in Shin Megami Tensei IMAGINE. Wearing his outfit grants players either Feast of the Earth Mother or Xeroth Beat, both almighty spins.

He is the only protagonist in the Shin Megami Tensei series to not use a COMP to summon demons.

It can be inspected that the Demi-Fiend's shadow in gameplay shows him in his human form with his hoodie, rather than his demon form or leather jacket.

The Demi-fiend's journey, struggles, and choices seem to be deeply influenced by Friedrich Nietzsche's philosophical novel "Thus Spoke Zarathustra" and the Demi-fiend seems to fulfill the role of the Ubermensch in each of his endings. Unlike other artistic portrayals, the Demi-fiend seems to be an accurate representation of the Ubermensch.[3][4]

In the DLC quest "Messiahs in the Diamond Realm" in Shin Megami Tensei IV: Apocalypse, the Demi-fiend is immune to all attacks barring Gun. It an be implied that he's still using the Masakados Magatama from his source game.

Demi-Fiend's death is the only one whose death is possible in the final battle for an ending in Nocturne; The True Demon Ending.